Crafting subdocuments with the aggregation framework

Hi,
I would like to use the aggregation framework for tasks like listing the most popular tags per day, unfortunately I can't do it without some client side logic because the aggregation framework allows to craft arrays but not subdocuments (objects).
This is a full explanation of what I would like to achieve:
http://stackoverflow.com/questions/36203290/mongodb-aggregation-framework-list-of-most-popular-tags-per-day